The present invention relates to an electromagnetic clutch including a leaf spring having a function of causing an armature hub to support an armature while biasing the armature away from a rotor, and a function of regulating the movement of the armature in a non-excited state.
A related electromagnetic clutch is disclosed in Japanese Utility Model Publication No. 61-37862 (literature 1). This electromagnetic clutch includes an annular rotor as an input-side rotary member. A field having an electromagnetic coil is inserted inside the rotor. An input shaft of a device to be driven is inserted into the axial portion of the rotor. An armature hub is formed on a distal end portion of the input shaft. A leaf spring extending in the radial direction of the rotor is fixed to the armature hub by rivets.
An armature is fixed to the end portion of the leaf spring on the outside of the rotor in the radial direction. The leaf spring biases the armature away from the rotor. The armature is formed into an annular shape when viewed in the axial direction of the input shaft. The armature is supported by the armature hub via the leaf spring, thereby being held in a position facing the end face of the rotor in the axial direction. When the electromagnetic coil is excited, the armature is magnetically attracted to the rotor against the spring force of the leaf spring.
The leaf spring includes an annular main body having an annular shape when viewed in the axial direction of the above-described input shaft, and a stopper projecting inside the annular main body. The annular main body is formed into a non-circular shape which is axially symmetrical with respect to a virtual central line extending in the radial direction of the rotor, when viewed in the axial direction of the input shaft. The annular main body includes a proximal end portion intersecting the virtual central line inside the rotor in the radial direction, and a free end portion intersecting the virtual central line inside the rotor in the radial direction. The proximal end portion is fixed to the armature hub by a rivet. The free end portion is fixed to the armature by a rivet.
The stopper includes a tongue projecting from the proximal end portion to the free end portion of the annular main body. Stopper rubber member is formed at the distal end portion of the tongue. When power supply to the electromagnetic coil is stopped in this related electromagnetic clutch, the armature is separated from the rotor by the spring force of the leaf spring, and abuts against the stopper rubber member and stops.
Since the stopper rubber member is formed in the stopper of the leaf spring, the outside dimension (outer diameter) of a flange of the armature hub in the radial direction can be made smaller than the inside dimension (inner diameter) of the armature in the radial direction. Consequently, an inexpensive lightweight electromagnetic clutch can be provided.
In this related electromagnetic clutch, however, if the input shaft of the device to be driven is locked by some cause, the armature may fall outside the electromagnetic clutch when power supply to the electromagnetic clutch is stopped. This is probably caused by the structure of the leaf spring as will be explained below.
When the input shaft is locked, an excess load is applied to the proximal end portion of the leaf spring and stress concentrates thereto, and cracking occurs in the periphery of a rivet hole of the proximal end portion. Then, the proximal end portion of the leaf spring is broken in the periphery of the rivet hole, and comes off the armature hub. Therefore, the proximal end portion of the leaf spring leaves the armature hub immediately after the input shaft is locked, and the armature and leaf spring rotate together with the rotor. After that, the armature and leaf spring leave the rotor when power supply to the electromagnetic clutch is stopped. Since, however, the outer diameter of the flange of the armature hub is smaller than the inner diameter of the armature, the armature is not supported by the flange of the armature hub. Consequently, the armature and leaf spring are spun off and fall outside the electromagnetic clutch as described above.
As described above, an inexpensive lightweight electromagnetic clutch can be provided by forming the stopper rubber member in the stopper of the leaf spring. On the other hand, if the proximal end portion of the leaf spring is broken in the periphery of the rivet hole, the rotating armature and leaf spring may be spun off and fall outside the electromagnetic clutch.